1. Field of the Invention
This invention relates to a package configuration of solar cells, particularly of Group II-VI compound semiconductor solar cells.
2. Description of the Prior Art
Recently, solar energy has been attracting attention as a promising substitute for existing energy sources. Various solar cells for converting radiant energy of sunlight to electric energy, which is more readily usable than any other energy, by virtue of the photoelectric effect of semiconductors have been developed and successfully put to actual uses. Solar cells are classified according to their constitution (crystallographic morphology) broadly under single crystals, polycrystals, and amorphous masses. They are broadly classified according to their material under Group VI elements, Group III-V compounds, and Group II-VI compounds. Among these solar cells, those formed on glass substrates by the screen printing method and the sintering method are capable of conversion efficiency of about 8%. A solar cell made of Group II-VI compound semiconductors forming a heterojunction between an n-type CdS film and a p-type CdTe film is receiving increased attention not only because it permits enlargement of surface area but also because it makes possible good cost reduction and has good mass-producibility.
FIG. 1 illustrates the basic configuration of this type of solar cell. The construction of this solar cell and a process for the production of this cell will be described below with reference to FIG. 1. A substrate 101 is made of ordinary alkali-free borosilicate glass. A sintered CdS film 102 destined to form the n-type material of a p-n heterojunction is obtained by screen printing on the glass substrate 101 a paste prepared by kneading a finely divided CdS powder containing 1 weight % of KCl and 6 weight % of CdCl.sub.2 as a fusing agent with a suitable amount of propylene glycol, drying the deposited paste, and firing the dried layer in an atmosphere of nitrogen containing a minute amount of oxygen at a temperature of about 700.degree. C. A sintered CdTe film 103 is obtained by kneading a finely divided CdTe powder with CdCl.sub.2 added thereto as a fusing agent and a suitable amount of propylene glycol also added thereto for forming a paste, screen printing the paste selectively on the sintered CdS film 102, drying the deposited paste, and firing the dried layer in an atmosphere of nitrogen at a temperature of about 630.degree. C. Numeral 104 designates an electrode formed on the sintered CdS film 102. This is a negative electrode (Ag-In layer) obtained by screen-printing an In-containing Ag paint and firing the deposited paint at about 490.degree. C. Numeral 105 is a carbon film formed on the sintered CdTe film 103. This is obtained by kneading a graphitic carbon powder containing about 100 ppm of Cu ions with an organic binder for forming a paste, screen-printing the paste, drying the deposited paste, and firing the dried layer in an atmosphere of nitrogen at a temperature of about 300.degree. C. by the printing and firing treatments, the Cu ions in the carbon film are diffused into the sintered CdTe film 103, and the sintered CdTe film acts as an acceptor. Numeral 106 designates a positive electrode (Ag layer) obtained by printing on the carbon film a conductive Ag paste prepared by dispersing a finely divided Ag powder in a synthetic resin such as epoxy resin and curing the deposited paste at about 150.degree. C., thereby forming an ohmic layer on the carbon film. An external conductor 107 issues from the electrode 104 (Ag-In layer) on the sintered CdS film 102 and an external conductor 108 issues from the electrode 106 (Ag layer) on the sintered CdTe film 103. These conductors can be electric wires made of copper and are joined at suitable locations to the respective electrodes with a conductive adhesive agent 109 prepared by dispersing a finely divided Ag powder in a synthetic resin such a epoxy resin. A cell protecting film 110 is provided by applying a thin coating of a synthetic resin such as epoxy resin or polyester resin on the surface of the cell having the foregoing structure for the purpose of protecting the cell surface against injury and providing electrical insulation for the cell. The process so far described is the basic procedure for the production of the CdS/CdTe heterojunction type solar cell by the screen printing technique. This solar cell is finished by being fabricated into the package configuration illustrated in FIG. 2 to be readied for shipment as a commercial product.
In the diagram of FIG. 2, 201 denotes the solar cell formed on a glass substrate 202 by the above-described printing and firing treatments. (The cross-section of the cell is similar to that of FIG. 1 and, therefore is illustrated here with some details omitted.) Denoted by 203 and 204 are positive and negative external conductor leads respectively connected to the cell. A backplate 205 is provided which is made of a metallic material such as aluminum or stainless steel. This backplate 205 functions as a structural support for mechanically carrying the solar cell which is on the glass substrate. These external conductor leads 203 and 204 are led out of the backplate through electric insulating sleeves 206 such as ceramic insulators extending through the backplate. Numeral 207 denotes a synthetic resin layer having rubbery elasticity filling the space between the solar cell on the glass substrate and the backplate. This resin can be silicone resin, polyurethane resin, copolymer resin of polyethylene and vinyl acetate, or butyral resin, for example. The resin layer which is between the solar cell on the glass substrate and the backplate serves to keep them joined to each other and, at the same time, to prevent the sintered layers of the cell from being penetrated by the humidity in the atmosphere and corrosive gasses to the fullest possible extent, which penetration can cause a change in properties of the cell. Numeral 208 designates a frame which is formed of a metallic material such as aluminum or stainless steel, and which acts as an additional structural support not only to strengthen the integrated unit of the glass substrate and the backplate by mechanically squeezing these members together circumferentially of the unit but also to facilitate the handling of the cell as a commercial product. It is combined with the unit by means of the aforementioned filler of resin material.
The solar cell formed of Group II-VI compound semiconductors as illustrated in FIG. 1 and FIG. 2 has to date been evaluated as promising, since it can be produced at low cost and with good mass producibility, because sintered films such as CdS and CdTe can be easily formed on substantially the entire surface of a glass substrate of a desired printable size by repetition of the relatively easy process of printing. While continuing a diligent study devoted to further improving practical solar cells as to quality and reliability and to developing a package configuration of solar cells capable of being mass produced at low cost, the inventors have found that such practical solar cells still suffer from many problems, such as high susceptibility to temperature changes, humidity, and insufficiency of mechanical strength required for commercial product handling. These problems may be summarized as follows:
(1) When a conventional solar cell is left standing outdoors for a long time, it is affected by humidity in the atmosphere and undergoes gradual deterioration of quality. Such a cell requires improvement particularly in properties which are affected by conditions of high temperature and high humidity. FIG. 3 represents a typical characteristic diagram showing the change of conversion efficiency with the passage of time. This diagram was obtained when a solar cell having a conventional package configuration as shown in FIG. 2 was subjected to a typical accelerated durability test in an atmosphere with a varying temperature of 80.degree. to 85.degree. C. and a varying relative humidity (RH) of 90 to 95%. In FIG. 3, the curve (b) represents the characteristic of conversion efficiency obtained when the sample was left standing in a dry atmosphere at 80.degree. to 85.degree. C. and the curve (a) the characteristic of conversion efficiency obtained when the sample was left standing in an atmosphere of 90 to 95% RH at 80.degree. to 85.degree. C. It is noted from the characteristic curves that the solar cell is stable with respect to the temperature, but it is susceptible of deterioration under the influence of humidity. Thus, for the solar cell to be suitable for practical use, this susceptibility to deterioration due to humidity must be overcome by some means or other.
(2) When the conventional solar cell is exposed to an abrupt change in external temperature, the difference between the thermal expansion coefficients of the solar cell element and the interposed layer of resin give rise to strain in the boundary between the solar cell and the interposed layer of resin, which strain can cause cracks in the surfaces of the sintered films of the cell and induce layer separation. Prevention of this deficiency necessitates selection of a proper material for the interposed layer and adoption of a method of incorporation of the layer.
(3) The glass substrate in the cell is required to be protected from such unwanted external forces as bending and twisting which occur during the handling of the solar cell as a commercial product. This can be done by improving the mechanical strength of the backplate and the frame. However, this improvement in mechanical strength should not add any weight or visible dimensions to the cell.
(4) To withstand an abnormal surge of current, such as a bolt of lightening, the cell and the backplate are required to possess a high breakdown voltage. In actual service, the cell should be able to withstand a surge voltage which exceeds 2 KV.
(5) The package should be so constructed that even when the glass substrate is broken, CdS, CdTe and other substances which are possible causes for environmental pollution will be prevented from being scattered from the housing.
(6) The solar cell is required to be constructed so that the production thereof can be easily automated for reducing the cost and enhancing mass producibility.